(Bug 5265) Add a "select-all" checkbox component

Given something with the appropriate HTML structure (see
dev/kitchensink for an example), style it and initialize the JS.
(Bug 5265) Separate out screenreader-friendly styling

Include for the elements you need by doing:

* @import "mixins/screenreader-friendly"
* .element-that-needs-to-be-hidden-visually-but-not-from-screenreaders {
	@extend %screenreader-friendly
  }